The present invention relates to polymer concrete structures in the form of vessels, cells or other containers or components of a type that may be used, for example, for containing or use with acid solutions, especially heated acid solutions e.g. electrochemical cells.
Polymer concrete refers to compositions that are formed from thermosetting polymers and aggregates, especially in which the aggregate is particulate siliceous fillers e.g. sand, gravel, quartz stones and the like.
Polymer concrete is particularly intended for use in the forming of a variety of structures that are exposed to corrosive environments and/or which are subject to abrasive environments. The corrosive environments may be atmospheric conditions in which the structure would be exposed to acids that exist in the atmospheric environment. In other embodiments, the polymer concrete is intended for use in the formation of vessels that are intended to contain corrosive chemicals, for instance acids.
One end-use for polymer concrete compositions is in the field of electrolytic recovery of metals from corrosive metal-bearing acid solutions. Techniques for the recovery of metals from ores or concentrates frequently involve the use of electrolysis, often using warm or hot acidic solutions. The warm or hot acidic solutions are contained in vessels, known as cells, that have a plurality of rows of electrodes, which are alternately anodes and cathodes. Electrolytic deposition of the metal is effected from the electrolyte onto the cathode. The electrolyte is almost invariably an acid solution of a type which can be highly corrosive to materials from which the container or cell is formed.
Cells were traditionally produced from concrete, with a chemically-resistant non-bonded liner incorporated inside the tank. However, such liners did not provide long term protection for the concrete. Damage to the liner e.g. cracks and holes, resulted in penetration of hot acid through the liner to the concrete, and the likelihood of catastrophic deterioration of the concrete cell. Maintenance costs were very high.
Concrete cells with liners have been replaced with cells formed from polymer concrete compositions of vinyl ester polymers and aggregate blends, the latter normally being siliceous material in a particulate form e.g. sand or gravel. These polymer concrete compositions typically have 10-12% by weight of polymer. While cells formed from vinyl ester resins/aggregate blends are a significant improvement over lined concrete cells, it was found that cracks occur in the manufacture of the cell i.e. in the so-called pre-cast product, necessitating repair and complete coating of both interior and exterior surfaces with a high build/high polymer content layer before the cell can be released for use in an electrochemical process.
The tendency for cells made from vinyl ester resin/aggregate blends to crack during casting results in the need to provide internal coatings for such tanks, and/or to effect repairs on site after installation but before use of the cell. The need to provide coatings and repairs is both time consuming to the manufacturer of the cell and to the user of the cell, and an added expense in the manufacture and supply of such cells. While these cells represent a significant improvement over previous cells, the need to conduct repairs prior to use partially defeats the intent and gains to be obtained from use of polymer concrete compositions.
In addition to the use of polymer concrete compositions in electrochemical cells, there are other structures that require protection against acidic environments or abrasive conditions, in which traditional concrete is susceptible to the environment and where additional protective layers are required or could be beneficial. Such other structures could include beams, channels, curbs, drains, chutes, pipes, floors and structures that require chemical and abrasive protection, compared to traditional concrete.
Polymer concrete compositions that are intended to be used in environments that are exposed to corrosive chemical attack are known. For instance, U.S. Pat. No. 4,621,010 and related EP 0 170 740 are directed to composite materials suitable for use in making containers or structures exposed to corrosive chemical attack, which are formed by mixing a synthetic resin material with two different hardeners and employing a filler of particulate siliceous material e.g. sand, gravel, quartz stones or the like. A skin coat formed from the same resin but using a filler with a fine particle size of less than 0.5 mm may be added, which permits use of larger particles, such as 6 mm and above, in the formation of the cell. Typical sizes of the filler were stated to be about 40% by weight of total filler in the range 0.5-1 mm, with other fractions including about 15% by weight in the range 1-1.75 mm and a further 15% by weight in the range 1.75-3 mm. It was found that larger particles, up to 6 mm, imparted strength to the structures that had been formed.
A polymer concrete structure has now been found that is resistant to acid solutions.
An aspect of the present invention provides a structure for resisting acid solutions, said structure being formed from a filled thermosetting polymer composition comprising a mineral filler in an amount of at least 92% by weight and not more than 8% by weight of thermosetting polymer, said composition containing a wetting agent, said composition having a co-efficient of thermal expansion that is less than 15xc3x9710xe2x88x926 in/in/xc2x0 F., the filler being a mineral particulate filler with generally rounded edges and the thermosetting polymer composition having a density that is at least 95% of the theoretical density for said filler and polymer, the filler being comprised of at least 50% by weight of a particle size that is greater than 6 mm, at least 70% by weight of a particle size that is greater than 2.4 mm, and at least 85% by weight of a particle size that is greater than 0.4 mm, the thermosetting polymer being obtained by reaction of (a) an epoxy resin formed from at least one of Bisphenol A and Bisphenol F with (b) an amine selected from at least one of an aliphatic and a cyclo-aliphatic amine.
A further aspect of the present invention provides a vessel for resisting acid solutions, said vessel being formed from a filled thermosetting polymer composition comprising a mineral filler in an amount of at least 92% by weight and not more than 8% by weight of thermosetting polymer, said composition containing a wetting agent, said composition having a co-efficient of thermal expansion that is less than 15xc3x9710xe2x88x926 in/in/xc2x0 F., the filler being a mineral particulate filler with generally rounded edges and the thermosetting polymer composition having a density that is at least 95% of the theoretical density for said filler and polymer, the filler being comprised of at least 50% by weight of a particle size that is greater than 6 mm, at least 70% by weight of a particle size that is greater than 2.4 mm, and at least 85% by weight of a particle size that is greater than 0.4 mm, the thermosetting polymer being obtained by reaction of (a) an epoxy resin formed from at least one of Bisphenol A and Bisphenol F with (b) an amine selected from at least one of an aliphatic and a cyclo-aliphatic amine.
The present invention utilizes a polymer concrete composition which is a filled thermosetting polymer composition having a mineral filler content of at least 92% weight. Conversely, the composition contains not more than 8% by weight of the thermosetting polymer. In preferred embodiments of the invention, the polymer concrete composition contains 92-94% by weight of the mineral filler, and correspondingly 6-8% by weight of the polymer.
The thermosetting polymer may be varied depending on the particular conditions of use of the resultant structure e.g. the chemical or abrasive environment that the structure would encounter during normal use. Preferred examples of the thermosetting polymer are formed from epoxy resins that are based on at least one of Bisphenol-A and Bisphenol-F and at least one of an aliphatic and a cycloaliphatic amine. Examples of such amines are known. Epoxy resins based on Bisphenol-A are typically a reaction product of bisphenol-A and epichlorohydrin, which gives the diglycidyl ether of bisphenol-A. The equivalent weight is preferably 182-192, and such a resin has a viscosity at 25xc2x0 C. of 11000-14000 mPaxc2x7s. Epoxy resins based on Bisphenol-F are typically a reaction product of bisphenol-F and epichlorohydrin. The equivalent weight is preferably about 160, and such a resin has a viscosity at 25xc2x0 C. of about 3500 mpaxc2x7s. Examples of aliphatic amines are triethylene tetramine and diethylene triamine. Examples of cycloaliphatic amines are isophorone diamine and diaminocyclohexane.
The filled thermosetting polymer composition of the invention is characterized by having a co-efficient of thermal expansion that is less than 15xc3x9710xe2x88x926 in/in/xc2x0 F. In preferred embodiments of the invention, the co-efficient of thermal expansion is less than 12xc3x9710xe2x88x926 in/in/xc2x0 F.
The filler that is used in the filled thermosetting polymer compositions is preferably a mineral particulate filler that has generally rounded edges. In particular, the filler should not be characterized by sharp angular edges, as such a profile provides a greater surface area for polymer xe2x80x9cwettingxe2x80x9d and greater void formation and such profiles are generally lower in impact/compression load resistance. As illustrated herein, filler with sharp angular edges also has detrimental effects on the flow of the compositions, compared to filler with round edges. While the filler may have a variety of shapes, of a random nature, such shapes should be characterized by generally rounded edges. In particular, the particulate has edges that are smooth. The filler is further particularly characterized by being comprised of at least 50% by weight of a particle size that is greater than 6 mm, at least 70% by weight of a particle size that is greater than 2.4 mm and at least 85% by weight of a particle size that is greater than 0.4 mm.
The composite structure formed from the thermosetting polymer composition should have a density that is at least 95% of the theoretical density for that particular combination of filler and polymer. The theoretical density may be calculated on the basis of cured polymer that does not contain filler and the amount of filler, calculated in terms of volume and weight. The requirement that the composition have a density that is at least 95% of the theoretical density, and more particularly at least 97.5% of the theoretically density, is an indication of the degree of voids within the composite structure. The presence of voids is an indication of potential defects within the composite structure, and thus the level of voids should be minimized. In embodiments in which the polymer concrete composition is to be bonded to a layer of concrete, such a density is comparable to that of concrete, and represents minimal air entrapment and high impermeability.
The composite structure may be either pigmented, or more preferably clear to permit visual observation and for confirmation of a consistent distribution of aggregate within the polymer.
In addition, the compositions of the present invention exhibit a low peak exotherm to minimize contraction on cooling. High exotherm temperatures tend to result from compositions with higher polymer content and subsequent contraction upon cooling can result in formation of cracks and development of stress within the polymer concrete. In particular, the compositions of the present invention exhibit a peak exotherm temperature that is not greater than 25xc2x0 C. higher than the ambient temperature at which curing occurs. It is further preferred that the curing occur at a temperature in the range of 15-35xc2x0 C. Such curing must occur in the absence of external cooling or heating, because such cooling or heating tends to create areas of stress within the structure, which may lead to cracks during use or transportation. The low peak exotherm temperature resulting from the present invention effectively eliminates cracking on contraction of the structure on cooling.
The compositions of the present invention contain at least one wetting agent, and preferably a mixture of wetting agents. Examples of such wetting agents include saturated polyesters with acid groups, titanate coupling agents and functional silanes, especially mixtures thereof.
In preferred embodiments of the present invention, the filled thermosetting polymer composition exhibits a slump diameter of greater than 18 cm.
As used herein, slump reflects the ability of the composition to flow freely under the influence of gravity, and is measured by placing a 0.28 liter sample into an open cylinder with a diameter of 7.2 cm on a smooth flat surface and then removing the cylinder to allow the composition to flow freely. The smooth flat surface should be formed from the material of the mould that is to be used, or have similar surface characteristics. The slump is the average diameter of the resultant mass of the composition after curing.
Structures are formed from the filled thermosetting polymer compositions described herein by techniques that are known. In particular, the filled thermosetting composition is poured into a mould of the desired shape, and permitted to cure at ambient temperature for a period of time that depends on the temperature, thickness and other factors but which is usually a period of 1-24 hours.
In the present invention, the filled thermosetting composition is used for the entire cell structure. Electrochemical cells may be formed using the epoxy polymer compositions to obtain cells that do not exhibit cracking on casting and cooling, or require minimal repair for cracks and other defects, particularly compared with existing electrochemical cells formed from filled vinyl ester polymer compositions. The advantage of using the epoxy filled compositions of the invention in the manufacture of electrochemical cells is exemplified below.
In one embodiment of the use of the present invention, an electrochemical cell is formed for use in the electrowinning of metals from acid solution, especially 14-22% w/w sulphuric acid solution at temperatures of 45-70xc2x0 C. In particular embodiments, the solution contains about 18% w/w sulphuric acid and the temperature is 60-65xc2x0 C.